Electronic documents are an alternative to paper documents. An electronic signature functions as an indication that a person adopts the content of the electronic document. Electronic documents are often exchanged between a sender and a signer using different computers that are connected via the Internet or another communication network. Since there is no physical copy of the document, certain measures must be undertaken to ensure that the integrity and confidentiality of each document is maintained when the document is transmitted from one party to another. An intermediate, third party service may be used to obtain electronic signatures. For example, a sender provides an unsigned copy of a document to the third party service, which in turn provides the document to the signer. The signer then applies an electronic signature to the document. The signed document is then returned to the sender via the third party service. With some existing techniques, the third party service reads the document as it is transmitted between the parties, which can potentially compromise the security of sensitive information that is intended to remain confidential. Thus, there is a need for improved techniques for providing an electronic signature service where the content of an electronic document to be signed remains unknown to the electronic signature service.